1. Field of the Invention
The present invention relates to a method of reading out signals for a solid-state imaging device used in a video camera, image pick-up device, or the like.
2. Description of the Prior Art
Nowadays, two-dimensional solid-state imaging devices using charge transfer means are widely used in broadcast and home video cameras and the like. To produce images using such two-dimensional solid-state imaging devices and conforming to the scanning system of television, 2:1 interlaced scanning must be performed in the two-dimensional imaging devices to conform to the 2:1 interlaced scanning system of television in which two fields are traced during each frame period. The following describes a method of reading out signals for a solid-state imaging device in which 2:1 interlacing in such a two-dimensional solid-state imaging device is achieved by a field-integration method.
FIG. 6 shows a schematic diagram of an interline transfer charge-coupled (CCD) area/image sensor as an example of a two-dimensional solid-state imaging device in which 2:1 interlacing is achieved by a field-integration method. In the area/image sensor shown, a plurality of photoelectric convertors 1, 1, . . . each comprising a photodiode, an MOS transistor, etc. are arranged in a lattice-like array (i.e., an array of rows and columns) corresponding to the pixels of the television display system. When a gate pulse is applied to a transfer gate 2 during the vertical blanking period, charges generated by the photoelectric convertors 1, 1, . . . in proportion to the light intensity are simultaneously transferred into corresponding storage sites 3a, 3a, . . . in vertical transfer sections 3 consisting of CCDs arranged in the vertical direction alternately with the vertical arrays of photoelectric convertors 1, 1, . . .. After that, photoelectric conversion is performed once again by the photoelectric convertors 1, 1, . . ., so that the next set of signal charges proportional to the light intensity are accumulated.
During each horizontal blanking period, the signal charges transferred from the photoelectric convertors 1, 1, . . . to the corresponding storage sites 3a, 3a, . . . in the vertical transfer sections 3 are transferred line by line, as shown by arrow (A), into corresponding storage sites (not shown) in a horizontal transfer section 4 consisting of a CCD array. The signal charges for one scanning line that have been transferred into the horizontal transfer section 4 are then clocked horizontally, as shown by arrow (B), toward the final stage transfer gate 6 by the clock timing shown in FIG. 8. The signal charges for one field are thus output line by line from the final stage transfer gate 6 to a charge detector 5, and the signal sensed by the charge detector 5 is amplified by an amplifier 7 to form a standard video signal.
FIG. 7 is a cross sectional view of the horizontal transfer section 4, transfer gate 6, and charge detector 5 of FIG. 6, with applied potentials indicated.
The above-mentioned field-integration method achieves 2:1 interlacing by reading out signals in the following manner. In the field-integration method, all the signal charges generated by the photoelectric convertors (hereinafter referred to as pixels) to produce one frame are read out during one field period. To achieve this, signal charges from two vertically adjacent pixels 1a and 1b shown in FIG. 6 (hereinafter simply referred to as "pixel pair") are combined and simultaneously read out into one storage site 3a in the adjacent vertical transfer section 3. For example, for an odd-numbered field, signal charges from a pixel 1a.sub.1 and a pixel 1b.sub.1 are combined and simultaneously transferred into one storage site 3a.sub.1 in the adjacent vertical transfer section 3 (as shown by solid arrows in FIG. 6); on the other hand, for an even-numbered field, signal charges from a pixel pair offset by one pixel in the vertical direction, i.e. the pixel 1b.sub.1 and a pixel 1a.sub.2, are combined and simultaneously transferred into the storage site 3a.sub.1 (as shown by dotted arrows in FIG. 6). As a result, as shown in FIG. 9, apparent space sampling points for an odd-numbered field are each taken at the middle point (indicated by a circle numbered "1") between two vertically adjacent pixels 1a and 1b constituting each pixel pair in areas 31 defined by thin solid lines, while apparent space sampling points for an even-numbered field are each taken at the middle point (indicated by a square numbered "2") between two vertically adjacent pixels 1b and 1a constituting each pixel pair in areas 32 defined by thin dotted lines.
The space sampling points are thus arranged in a lattice-like pattern, as shown in FIG. 9, at the same intervals at which the pixels 1, 1, . . . are arranged.
As shown in FIG. 8, in the method of reading out signals for the above solid-state imaging device, the timing of both a final gate transfer pulse signal .phi..sub.H1 ' and a reset pulse signal .phi..sub.R is set so as to synchronize with a horizontal CCD drive pulse signal .phi..sub.H1 and a horizontal CCD drive pulse signal .phi..sub.H2 with the same clock cycle. Therefore, every space sampling point is positioned at the middle point between two vertically adjacent pixels constituting each pixel pair. That is, the signal charge obtained from a given space sampling point represents combined signal charges of two vertically adjacent pixels.
However, with the recent advances toward increased pixel count and miniaturization of optics, the unit pixel area has been reduced to such a degree that the sensitivity characteristics of photoelectric convertors under a low light intensity are now nearing the theoretical point where shot noise associated with photoelectric conversion will become dominant. Therefore, the above method of reading out signals for a solid-state imaging device has the problem that since the signal charge of one space sampling point is obtained by combining signal charges of two vertically arranged pixels 1 and 1, the absolute signal charge amount is low per space sampling point, resulting in a drop in the signal-to-noise ratio under a low light intensity, and hence degradation in sensitivity characteristics.